The present invention relates to magnetic storage devices and, more particularly, to computer disk drives. More specifically, the present invention relates to determining a data density at which to store data on a disk surface by measuring thermal decay rates for the disk surface for various data densities.
Computer disk drives store digital information on magnetic disks which are coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Typically, the digital information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk.
More specifically, during operation of a conventional disk drive, a magnetic transducer is placed above a desired track of the disk while the disk is spinning. Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track which constitute the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly-timed digital signal that can be recognized by a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Typically, transducers include separate elements for reading and writing. Such transducers are known as dual element heads and usually include a magneto-resistive (MR) read element or giant magneto-resistive (GMR) read element for performing the read function.
Dual element heads are advantageous because each element of the transducer can be optimized to perform its particular function. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter signals from the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface with no loss of read performance.
MR read elements generally include a strip of magneto-resistive material that is held between two magnetic shields. The resistance of the magneto-resistive material varies almost linearly with applied magnetic field. During a read operation, the MR strip is held near a desired track, with the varying magnetic field caused by the magnetic transitions on the track. A constant DC current is passed through the strip resulting in a variable voltage across the strip. By Ohm""s law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host. GMR read elements operate in a similar manner.
FIGS. 1(a)-1(e) are simplified diagrammatic representations which illustrate how data is written as transitions on a disk surface and how the transitions are read from the disk surface as data. As background, a transition is where the magnetization in the disk media changes. In general, there are two types of transitions possible; that is, where south poles face south poles and where north poles face north poles.
FIGS. 1(a)-(c) illustrate the write process in simplified form. Specifically, FIG. 1(a) illustrates a data sequence in the form of xe2x80x9conesxe2x80x9d and xe2x80x9czeros,xe2x80x9d which is to be stored on the disk media. FIG. 1(b) illustrates the write current in the write coil for one method of storing the data sequence. In such method, the current through the write coil is reversed at each xe2x80x9conexe2x80x9d and remains the same at each xe2x80x9czeroxe2x80x9d (see FIGS. 1(a) and 1(b)). Consequently, as the disk media is rotated under the write head, the disk media is magnetized as shown in FIG. 1(c). It should be noted that magnetic transitions occur at each xe2x80x9conexe2x80x9d and not at each xe2x80x9czero.xe2x80x9d It should also be noted that FIG. 1(c) represents the magnetization of the media for a portion of a track, which is shown in a linear rather than arcuate shape, as will be understood by those skilled in the art.
FIGS. 1(d) and 1(e) illustrate the read process in simplified form. As mentioned above, as the disk media is rotated under the read head, a constant DC current is passed through the MR strip in the read head. The magnetic transitions stored in the disk media cause the magnetic field applied to the MR strip in the read head to vary, as shown in FIG. 1(d). Since the resistance of the magneto-resistive material varies almost linearly with applied magnetic field, the varying magnetic field caused by the magnetic transitions on the disk media results in a variable voltage across the strip. By Ohm""s law (i.e., V=IR), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track, as shown in FIG. 1(e). The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
The amount of information capable of being stored on a disk surface is determined, in part, by the minimum size of individual transitions. As is known to those skilled in the art, the minimum size of individual transitions is based (among other things) upon the grain size of the magnetic material forming the magnetic layer of the disk surface. In order to increase the amount of information capable of being stored on the disk surface, disk manufacturers have been continuously reducing the grain size of the magnetic material and, hence, have reduced the minimum size of individual transitions. For the magnetic layer of the disk, the remnant magnetization-thickness product has also been reduced to achieve higher linear densities and enhanced writer performance. Most of this reduction has been achieved by reducing the thickness of the magnetic layer of the disk, and hence, the grain thickness, which reduces the grain size.
Traditionally, about 500 to 1000 grains of magnetic material were required to store a bit of information. However, at present, a transition may be stored in about 250 grains of magnetic material, assuming a density of 25 Gb/in2. It is expected that the number of grains of magnetic material required to store a bit of information will continue to decrease over time. To reduce transition noise and increase the number of grains in a transition, both the diameter of the grains and the separation between the grains have been decreased. In fact, the diameter of the grains has decreased from approximately 15 nm down to approximately 9-10 nm. This has driven disk vendors to produce disks with smaller grain volumes.
As will be understood by those skilled in the art, each grain has a certain magnetic anisotropy energy associated with it. More specifically, the anisotropy energy of a grain is a fixed amount of energy required to xe2x80x9choldxe2x80x9d a stored direction of magnetization in the magnetic material. As grain sizes have been reduced, the anisotropy energy associated with each grain has been reduced. In fact, the anisotropy energy of each grain has been reduced such that it is comparable to the ambient thermal energy in the disk drive. Consequently, the thermal energy in the disk drive randomly excites grains in the magnetic material causing changes in the direction of magnetization of the magnetic material over time. Ultimately, if a threshold number of grains change their direction of magnetization, information stored on the disk may be lost. This phenomenon is known as the superparamagnetic effect (or thermal decay).
In other words, the superparamagnetic effect is a thermal relaxation of information stored on the disk surface. Because the superparamagnetic effect may occur at room temperature, over time, information stored on the disk surface will begin to decay. Once the stored information decays beyond a threshold level, it will be unable to be properly read by the read head and the information will be lost.
More specifically, the superparamagnetic effect manifests itself by a loss in amplitude in the readback signal over time. Accordingly, this causes the bit error rate (BER) to increase. As is well known, the BER is the ultimate measure of drive performance in a disk drive.
In general, a certain number of bit errors may be corrected by a disk drive""s error correction code (ECC). However, as is well-known, ECC information adds to the overall overhead of a disk drive, which limits the amount of information that can be stored on a disk surface. Accordingly, disk drives exhibiting high bit error rates that require correction by a large amount of ECC overhead are generally disfavored.
In designing disk drives for sale to consumers, disk drive manufacturers determine the storage capacity of a particular model of a disk drive in advance of its construction. Furthermore, the storage capacity of each disk surface in the disk drive is also determined in advance of the disk drive""s construction.
In ascertaining the thermal decay rate for disk surfaces in a drive, disk drive manufacturers have traditionally measured only a representative sample of the disk surfaces prior to installation into the disk drive. The inventors of the present invention have recognized that this approach is problematic because thermal decay rates vary from disk surface to disk surface due to the varying properties between disk surfaces. In fact, properties of disk surfaces may vary, not only between different disk manufacturers, but also between the same disk manufacturer. For example, the grain size and anisotropy energies of the magnetic material may vary between disk surfaces of the same manufacturer.
The inventors of the present invention have also recognized that the thermal decay rate is dependent, in part, on the density at which data is stored on a disk surface (i.e., the data density). If the data density for each of the disk surfaces is determined prior to manufacture, then there is a potential that one or more of the disk surfaces will have an unacceptable thermal decay rate after manufacture, which may lead to drive failures after drives are sold to consumers.
Accordingly, it would be advantageous to measure the thermal decay rate of one or more disk surfaces of a disk drive, after its construction, at one or more data densities. Furthermore, it would be advantageous to select the data density of one or more disk surfaces based upon measured thermal decay rates from the disk surfaces. Even further, it would be advantageous to select the data densities of the disk surfaces such that the overall storage capacity of the disk drive meets or exceeds a predetermined storage capacity.
The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.
In one embodiment, a method for selecting data densities on disk surfaces in a disk drive based upon measured thermal decay rates of the disk surfaces during a self-test procedure is disclosed. In such method, a disk drive is provided which includes, at least a first disk surface and at least a first head. A first data pattern having a first data density and a second data pattern having a second data density are written onto the first disk surface using the first head. The first and second data patterns are read from the disk surface using the first head upon expiration of at least a first predetermined time interval. The first thermal decay rate associated with the first data pattern and the second thermal decay rate associated with the second data pattern are calculated based at least upon the information respectively read from each of the data patterns following the expiration of the first predetermined time interval. A determination is made as to whether the first thermal decay rate associated with the first data pattern satisfies a first thermal decay rate requirement and whether the second thermal decay rate associated with the second data pattern satisfies a second thermal decay rate requirement. A density at which to record data on the first disk surface is selected based upon whether the first thermal decay rate meets the first thermal decay rate requirement and whether the second thermal decay rate meets the second thermal decay rate requirement.